Dan Dark (ROBERT DOWNEY, JR.) is a crime novelist who's hospitalized due to a severe case of psoriatic arthropathy, a crippling disease that's left his skin horribly scarred and his hands curled up into useless appendages. While the hospital's Chief of Staff (ALFRE WOODARD) and especially Nurse Mills (KATIE HOLMES) care for his body, staff psychiatrist Dr. Gibbon (MEL GIBSON) tries to figure out what's swirling about in the bitter man's head.
Beyond memories of his mother, Betty Dark (CARLA GUGINO), having an affair with her husband's coworker and then she and young Dan leaving home and setting off for another city, the writer has concocted an imaginary world. There, it's the 1950s and he's a singing gumshoe hired by Mark Binney (JEREMY NORTHAM) to investigate the murder of Nina (ROBIN WRIGHT PENN) by two hoods (ADRIEN BRODY & JON POLITO)
Not surprisingly, she looks quite a bit like Dan's real estranged wife, Nicola (ROBIN WRIGHT PENN), who's attempting, along with her lover (JEREMY NORTHAM), to steal his screenplay adaptation of his novel, The Singing Detective.
Confronted by childhood memories of Dan's past, imagined scenes from his screenplay and hallucinations featuring full blown musical numbers, Dr. Gibbon tries to sort out the convoluted psyche that simultaneously drives yet stymies Dark's state of mind and being.